Visitors to the Palo Alto Baylands Nature Preserve may notice the familiar sight of shorebirds and marsh grasses but not immediately recognize that a new form of flood infrastructure has been planted into the landscape. Rather than a vertical embankment of rock and concrete, the Palo Alto project is a horizontal levee — a gently sloped, densely vegetated bank meant to mimic natural estuary processes while providing flood protection and habitat.
What a horizontal levee is and why it matters
Traditional levees rely on steep faces and hard armoring to block storm waves and high tides. The Palo Alto design takes a different approach: it spreads elevation gain over a wider area and is planted with native vegetation so that waves break on the slope much as they would on a natural shoreline. The result is intended to both reduce flood risk and support wildlife.
The pilot was seeded with more than 35 native plant species, and project engineers say the dense vegetation plays a key role in dissipating wave energy. As Mark Lindley, the lead engineer on the Palo Alto living levee, explained, "By having this flat, gradual slope with really dense vegetation on it instead of having rock, the waves break on that slope like they do on a beach."
Engineered connections: wastewater, filtration and nutrient removal
Unlike many nature-based projects that rely solely on rainfall and tidal action, the Palo Alto horizontal levee is hydrologically connected to a nearby treatment facility. Beneath the planted soils and gravel, plumbing links the levee to the Palo Alto Wastewater Treatment Center, allowing treated wastewater to irrigate the slope.
That intentional irrigation serves more than establishment needs for plants. As water passes through the vegetated layers, the vegetation and soils help remove nitrogen — a common constituent of treated wastewater that can fuel harmful algal blooms when it enters the Bay in excess. After passing through the levee, the filtered water then makes its way to the Bay.
Regional context and precedent
Palo Alto's pilot follows earlier Bay Area work in living levees. The Oro Loma Horizontal Levee, completed in 2017 near Hayward, was the first full-scale example in the region and helped inform subsequent designs. The Palo Alto project is notable for being the first in the Bay to be connected directly to the Bay hydrologically while using treated wastewater as part of its irrigation and filtration system.
Designers and planners say the approach is gaining traction across the Bay; there are roughly a dozen horizontal levees at different stages of planning or construction around the region. Proponents argue the combination of flood risk reduction and ecological benefits makes the design particularly well suited to the Bay Area's tidal wetlands and rising seas.
- Benefit: Wave energy dissipation through gradual slopes and dense vegetation.
- Benefit: Habitat creation and restoration with native plantings.
- Benefit: Water quality improvement via nitrogen removal prior to discharge to the Bay.
Questions and local impacts
While the pilot demonstrates how green infrastructure can serve multiple functions, it is not presented as a universal substitute for all traditional flood-control measures. Site-specific conditions — including tidal range, storm surge, land availability and maintenance needs — will determine where horizontal levees are appropriate. For Palo Alto residents and managers of the Baylands, the pilot offers a living test bed to observe plant survival, filtration performance and how the slope withstands wave action over time.
Because the project uses treated wastewater for irrigation and filtration, monitoring will be important to document nutrient removal rates and to ensure that the system performs as designed across seasons and storm events. The pilot is also expected to inform cost and maintenance questions for other jurisdictions considering similar nature-based defenses.
| Project | Location | Notable feature |
|---|---|---|
| Oro Loma Horizontal Levee | Hayward | Completed 2017; regional precedent |
| Palo Alto Horizontal Levee Pilot | Palo Alto Baylands | Hydrologically connected; irrigated with treated wastewater; >35 native species |
As sea-level rise and more intense storms reshape planning priorities across the Bay Area, the Palo Alto pilot represents a local experiment in blending engineered resilience with ecological restoration. Observers and local officials will be watching whether it can deliver on both counts: protecting low-lying lands from flooding while improving habitat and water quality for the Bay.
